1. Field of the Invention
The present invention relates to a process for reducing nitrogen oxides (NO.sub.x), in which at least nitrogen oxides contained in exhaust gases emitted from internal combustion engines of automobiles, etc., can be efficiently reduced, the amount of oxygen in exhaust gases being larger than the stoichiometric amount of oxygen required to oxidize components to be oxidized therein, such as hydrocarbons (HC), carbon monoxide (CO), hydrogen (H.sub.2), etc.
2. Description of Related Art
Carbon dioxide (CO.sub.2) which is contained in exhaust gases emitted from internal combustion engines of automobiles, etc. has recently become an issue in view of protection of global environment. As one of solutions to the problem, a so-called lean-burn engine has gained the increasing prospect, in which a fuel is lean-burned in an atmosphere of excess oxygen. In the lean-burn engine, high combustion efficiency of a fuel decreases the consumption of a fuel and thus suppresses the emission of CO.sub.2 which is a combustion product of a fuel.
A conventional three-way catalyst simultaneously oxidizes and reduces carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NO.sub.x) in the exhaust gas of a stoichiometric air-fuel ratio. However, the three-way catalyst does not exhibit sufficient ability to reduce nitrogen oxides in the exhaust gas which contains excess oxygen in the amount which is larger than a stoichiometric amount required to oxidize the components to be oxidized in the exhaust gas, i.e., in the oxygen rich atmosphere. For this reason, the improvement of a process for reducing nitrogen oxides has been sought in which the nitrogen oxides can be reduced even in the case that the exhaust gas contains excess oxygen.
As a process for reducing NO.sub.x in an exhaust gas containing excess oxygen, Japanese Laid-Open Patent Application (Kokai) No. 62-97,630 discloses a two-catalyst system. In this system, the exhaust gas flows into the first catalyst, one of two catalysts arranged in parallel, to adsorb NO.sub.x, and when the catalyst is saturated with adsorbed NO.sub.x and NO.sub.x adsorption capacity is decreased, the flowing of the exhaust gas into the first catalyst is stopped and simultaneously switched to the second catalyst, and a reducing agent such as hydrogen or the like is allowed to flow into the first catalyst to reduce the NO.sub.x adsorbed.
The present inventors previously developed a technique in which a catalyst for the oxidation of HC, CO and NO, products of incomplete combustion of a fuel, is provided upstream in an exhaust gas passage, a catalyst for reducing NO.sub.2 formed by the oxidation is provided downstream in the exhaust gas passage, and hydrogen gas (H.sub.2) is fed from upstream of the reduction catalyst to thereby reduce NO.sub.x effectively (Japanese Laid-Open Patent Application (Kokai) No. 5-168,856).
Further, as a catalyst which shows a relatively high ability to reduce nitrogen oxides even under the oxygen rich atmosphere, there were proposed a zeolite catalyst in which a transition metal such as copper (Cu) or the like is supported on zeolite by ion exchange as disclosed in Japanese Laid-Open Patent Application (Kokai) No. 1-139,145, and a platinum catalyst in which a noble metal such as platinum (Pt) or the like is supported on alumina (Al.sub.2 O.sub.3) as disclosed in Japanese Laid-Open Patent Application (Kokai) No. 5-168,860.
However, in the process of above-mentioned Japanese Laid-Open Patent Application (Kokai) No. 62-97,630, there is a problem that the device itself is complicated since the two exhaust gas passages are necessary and they have to be switched over according to the amount of the NO.sub.x adsorbed. Another problem is that when H.sub.2 is fed for regenerating the catalyst, the catalyst is rendered in a reductive atmosphere, so that NO.sub.x may react with H.sub.2 to form ammonia (NH.sub.3).
In the process of the above-mentioned Japanese Laid-Open Patent Application (Kokai) No. 5-168,856, where H.sub.2 is continuously fed, the larger the amount of H.sub.2 is, the more efficiently NO.sub.x is reduced. However, when the amount of H.sub.2 fed is too large, H.sub.2 is reacted with oxygen, and the temperature of the catalyst increases. Because the reaction of H.sub.2 and oxygen is accelerated, NO.sub.x reduction reaction is suppressed and H.sub.2 utilization efficiency decreases.
The above-mentioned zeolite catalyst in Japanese Laid-Open Patent Application (Kokai) No. 1-139,145 involves the following problems.
(1) The heat resistance is poor.
(2) Although hydrocarbons (HC) are used as a reducing agent of NO.sub.x, the hydrocarbons emitted in a non-reacted state are not good to the human body.
(3) When hydrocarbons in the exhaust gas or fuels such as a light oil, gasoline, etc. are used as a reducing agent, only limited kinds of hydrocarbons can be used as a reducing agent, since some hydrocarbons are easy to react with NO.sub.x, but others are difficult to react with NO.sub.x. Accordingly, the hydrocarbons used as the reducing agent cannot reduce NO.sub.x efficiently.